Communication systems with increased resistance to frequency-selective fading caused by multipaths by using multi-carriers like IEEE (Institute of Electrical and Electronics Engineers) 802.11a, which is a wireless LAN (Local Area Network) standard, are used mainly in the field of high capacity wireless communications.
As such a communication system, for example, OFDM (Orthogonal Frequency Division Multiplexing) has been known. One of characteristics of the communication system using the multi-carriers such as OFDM is that frequency selective fading generated in a multipath environment may be handled as flat fading in each carrier. Further, with the increase of communication capacity a wide band is used, thus it is another characteristic to provide the Rayleigh distribution in a frequency axis direction.
There are four types of modulation systems prescribed in IEEE 802.11a, namely, BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), 16QAM (Quadrature Amplitude Modulation), and 64QAM. By changing a coding rate of error correction, a desired communication speed may be selected adaptively among eight communication speeds depending on a channel state.
FIG. 14 is a schematic diagram of a communication image in a multipath environment. FIG. 14 is an example of mapping 16QAM on all carriers. A sub-carrier number α (SC[α] in FIG. 14) indicates a relatively favorable communication characteristic. A sub-carrier number β (SC[β] in FIG. 14) indicates a poor communication characteristic because it corresponds to a plunge of frequency. IEEE 802.1 is easily realized because the same modulation system is assigned to all sub-carriers. In the multipath environment; however, it is highly possible that a bit assigned to a plunged sub-carrier generates an error.
As described above, the communication system according to IEEE 802.11a may realize wide band transmission by providing multi-values such as 16QAM and 64QAM according to favorable communication environments such as when a distance between communication devices is short and when interference waves do not exist. In the multipath environment; however, a transmission SN of a particular carrier may occasionally be deteriorated. To compensate for the deterioration, receiving or transmitting diversity, a plurality of repeated transmissions, and error corrections have been performed. For example, Patent Document 1 discloses a technique for rotating a phase of a carrier when obtaining the diversity effect in a time direction.    Patent Document 1: International Publication No. WO2005/004367 pamphlet